1. Field of the Invention
This invention relates to flexible thermosensitive wires which are adapted for use in electrically heated warming devices such as electric blankets, electric sheets or bedcovers and the like.
2. Description of the Prior Art
Since electric blankets and the like electric personal warming devices are used in direct contact with the skin, there has been a high demand for laundering. To meet the demand, washable electric blankets has been developed, which make use of flexible thermosensitive wires using thermosensitive polymer materials such as polyamide compositions of low moisture absorption.
In order for one to subject an electric blanket to dry cleaning, the heater wire has to be resistant to solvent. U.S. Pat. No. 2,941,176 proposed a heater wire which had an insulative jacket made of a high molecular weight monomeric ester plasticizer, i.e., pentaerythritol ester. However, this wire has not been found satisfactory with respect to the solvent resistance. Japanese Laid-open patent application No. 60-89901 has proposed a flexible thermosensitive wire whereby a solvent resistant is imparted. In this flexible thermosensitive wire, a thermosensitive polymer layer is covered with a barrier layer for solvent made, for example, of ethylene-vinyl alcohol copolymers or highly crystalline polyolefins or a layer of a non-dissolving polymer material such as an ethylene-vinyl chloride copolymer or a polymer alloy of the copolymer with polyvinyl chloride.
Thermosensitive polymer materials of a low moisture absorption are used in these flexible thermosensitive wires, but the polymeric barrier materials or non-dissolving polymer materials are also used to cover the thermosensitive polymer material. Accordingly, release of moisture from the thermosensitive polymer material or a material inside the thermosensitive polymer material is prevented. This involves the problem that the recovery of the thermosensitive characteristic becomes very slow. When such a wire is applied as an electric blanket, the temperature of the blanket cannot be precisely controlled during use within a short period of time.